webadmin: adjust action table vertical scroll-bar

Action table vertical scroll-bar is cropped
when the items list is empty (Fedora19/Firefox21).

The issue was overlapping of the bottom table
over the header (which cropped the header
also when the table isn't empty).

Fixed by altering table's styles.

Screenshots-Before:
http://i.imgur.com/aP64ZzY.png
http://i.imgur.com/koRMhc1.png

Screenshots-After:
http://i.imgur.com/j7CjGN6.png
http://i.imgur.com/HjUPhu4.png

Change-Id: I8ed7202f2ff13ec5adf2c5de341f2b8223a85a63
Signed-off-by: Daniel Erez <derez@redhat.com>